In general, a particle beam therapy system is provided with a beam generation apparatus that generates a charged particle beam; an accelerator that is connected with the beam generation apparatus and accelerates a generated charged particle beam; a beam transport system that transports a charged particle beam that is accelerated by the accelerator so as to gain predetermined energy and then emitted; and a particle beam irradiation apparatus, disposed at the downstream side of the beam transport system, for irradiating a charged particle beam onto an irradiation subject, which is a diseased site of a patient or the like. Particle beam irradiation apparatuses are roughly divided into apparatuses utilizing a broad irradiation method in which a charged particle beam is enlarged in a dispersion manner by a scatterer, and the shape of the enlarged charged particle beam is made to coincide with the shape of an irradiation subject in order to form an irradiation field; and apparatuses utilizing a scanning irradiation method (the spot-scanning method, the raster-scanning method, and the like) in which an irradiation field is formed by performing scanning with a thin, pencil-like beam in such a way that the scanning area coincides with the shape of an irradiation subject.
In the broad irradiation method, an irradiation field that coincides with the shape of a diseased site is formed by use of a collimator or a bolus. The broad irradiation method is a most universally utilized and superior irradiation method where an irradiation field that coincides with the shape of a diseased site is formed so as to prevent unnecessary irradiation onto a normal tissue. However, it is required to create a bolus for each patient or to change the shape of a collimator in accordance with a diseased site.
In contrast, the scanning irradiation method is a high-flexibility irradiation method where, for example, neither collimator nor bolus is required. However, because these components for preventing irradiation onto not a diseased site but a normal tissue are not utilized, there is required a positional accuracy of beam irradiation that is the same as or higher than that of the broad irradiation method.
Patent Document 1 discloses the following invention whose objective is to reduce the effect of the hysteresis of a scanning electromagnet for scanning a charged particle beam and to realize high-accuracy beam irradiation. The invention disclosed in Patent Document 1 includes an irradiation management apparatus that controls a scanning electromagnet, based on the desired irradiation position coordinates of a charged particle beam, and a position monitor that measures the measured position coordinates of the charged particle beam. The irradiation management apparatus is provided with a command value generator that outputs a control input to the scanning electromagnet, based on the desired irradiation position coordinates and correction data generated on the basis of the measured position coordinates, measured by the position monitor in preliminary irradiation in which the excitation pattern of the scanning electromagnet is the same as that of a actual irradiation plan, and the desired irradiation position coordinates.
Accordingly, in the invention disclosed in Patent Document 1, the scanning-electromagnet excitation pattern of the preliminary irradiation is the same as that of the planned actual irradiation, and the control input to the scanning electromagnet is preliminarily corrected based on the result obtained in the preliminary irradiation; therefore, the effect of the hysteresis of the scanning electromagnet is eliminated, so that high-accuracy beam irradiation can be realized.